The present invention relates to an active matrix type liquid crystal display device incorporating switching elements, such as Thin Film Transistors (TFTs), and further relates to a manufacturing method of such a device.
FIG. 5(a) is a plan view showing a pixel region of an active matrix substrate incorporated in a conventional liquid crystal display device. As shown in FIG. 5(a), the active matrix substrate has a plurality of pixel electrodes 54 provided in a matrix form. Gate wirings 51 and source wirings 52 are provided around the pixel electrodes 54 so as to orthogonally cross each other. A TFT 53 is provided in neighborhoods of crossing points of the gate and source wirings 51 and 52 as a switching element connected to the pixel electrode 54 through a contact hole.
FIG. 5(b) is a cross-sectional view taken along line Bxe2x80x94B of the active matrix substrate incorporated in the liquid crystal display device shown in FIG. 5(a). As shown in FIG. 5(b), a gate electrode 61 branching off from the gate wiring 51 shown in FIG. 5(a) is provided on a transparent insulating substrate 60. A gate insulating film 55 is provided to cover the gate electrode 61. A semiconductor layer 64 is provided on the gate insulating film 55 above the gate electrode 61. A channel protection layer 65 is provided on the center of the semiconductor layer 64. Two n+ layers 66, respectively serving as a source area and a drain area, are provided so as to cover both ends of the semiconductor layer 64 and the channel protection layer 65, and to be separated from each other on the channel protection layer 65. The n+ layers 66 are connected respectively to the source electrode 62 branching off from the source wiring 52 and the drain electrode 63. An interlayer insulating film 59 is provided to cover the TFT 53 and the gate and source wirings 51 and 52 provided in this manner. The pixel electrode 54 is provided on the interlayer insulating film 59. The pixel electrode 54 is connected to the drain electrode 63 of the TFT 53 through the contact hole in the interlayer insulating film 59.
Finally, the manufacturing process of the conventional liquid crystal display device becomes complete with sealing liquid crystal 58 between a TFT substrate 70 configured in the above manner and an opposite substrate 71 equipped with an opposite electrode 56. Here, spacers 57 are sandwiched between the TFT substrate 70 and the opposite substrate 71 to maintain a predetermined space therebetween (disclosed in Japanese Laid-Open Patent Application No. 61-156025/1986 Tokukaishou 61-156025).
Polyimide resin is used as the interlayer insulating film 59 in the conventional liquid crystal display device disclosed in the above laid-open patent application. However, other highly transparent materials, such as acrylic resin, polystyrene and polyester, are also generally used.
Although no disclosure is made about the spacers 57, plastic beads and hard materials, such as glass, are usually used. The plastic bead spacer is generally made of polyimide, epoxy and polystyrene.
Nevertheless, if the above-mentioned acrylic resin is used as the interlayer insulating film 59 and the spacers 57 are made using epoxy resin, Newton rings are observed in a lighting test after injecting and sealing the liquid crystal (Newton rings are a series of circular bright and dark bands, which look like a wave pattern created on water surface by a stone dropping into the water). Especially the phenomenon frequently occurs in the sealing portion, and the rings were even more clearly observed with a liquid crystal display device incorporating an interlayer insulating film in an underlayer of the sealing portion. Consequently, such a liquid crystal display device has problems of a high defective ratio and low reliability.
An object of the present invention is to provide a liquid crystal display device which does not cause Newton rings to occur and which has a low defective ratio and good reliability.
As a step toward achieving the above object, the inventors of the present invention conducted researches to pinpoint causes of Newton rings, and found out that if acrylic resin is used as an interlayer insulating film and epoxy resin is used as spacers as in a conventional liquid crystal display device, the spacers press a pixel electrode, sink into the interlayer insulating film, and thus change the space between a TFT substrate and an opposite substrate, thereby causing Newton rings to occur.
On the contrary, in order to achieve the above object, a liquid crystal display device in accordance with the present invention is configured in a preferred embodiment so that the spacers do not sink into the interlayer insulating film by optimizing materials of the interlayer insulating film and spacers. As a result, various problems with the liquid crystal display device using the resin-made interlayer insulating film are successfully solved.
Specifically, in a first preferred embodiment, the liquid crystal display device includes: a liquid crystal layer; a first substrate provided on one side of the liquid crystal layer; a second substrate provided on the other side of the liquid crystal layer; and a sealing member for sealing the liquid crystal layer between the first and second substrates. The liquid crystal layer has therein at least one first spacer for maintaining a thickness of the liquid crystal layer. The second substrate has: an interlayer insulating film of the same hardness with the first spacer or a greater hardness than the first spacer; a pixel electrode disposed on one side of the interlayer insulating film; at least one switching element disposed on the other side of the interlayer insulating film for controlling a display state of a pixel; and a base substrate facing the first substrate through the liquid crystal layer and the interlayer insulating film.
With the above configuration, the hardness of the interlayer insulating film is either the same as or greater than the hardness of the first spacer. Therefore, the first spacer can be prevented from sinking in the interlayer insulating film surface. Newton rings can be thus prevented from occurring, which improves reliability and defective ratio in manufacture of the liquid crystal display device. Moreover, even if pressure is applied by, for example, a user wiping the liquid crystal display device surface after the liquid crystal display device is manufactured, the space between the substrates are kept constant. The liquid crystal display device with high display quality is obtained for these reasons.
While the interlayer insulating film is formed to have a predetermined hardness in the above liquid crystal display device, an interlayer insulating film of a second preferred embodiment is made of elastic body. The second preferred embodiment is configured in the same manner as the first preferred embodiment, except that a different material is used for the interlayer insulating film. With this configuration, if pressure is applied to the display surface of the liquid crystal display device, the first spacer sinks in the interlayer insulating film surface and the space between the substrates changes temporarily. Nevertheless, as the pressure is removed, resilient power occurs and the space recovers to the predetermined space. Therefore, the space between the substrates of the liquid crystal display device are kept constant. The liquid crystal display device with good reliability, a low defective ratio and high display quality can be provided for these reasons in the same manner as in the first preferred embodiment.
In either of the liquid crystal display devices of the first and second preferred embodiments, the interlayer insulating film is preferably made of resin having a transmittance in the visible ray area of not less than 90%. With this configuration, especially if the interlayer insulating film is incorporated in a transparent type liquid crystal display device, it is possible to obtain beautiful image display with no coloring. Moreover, since transmittance can be prevented from deteriorating, it is possible to restrain increase of power consumption of the backlight.
Incidentally, in the two embodiments above, the material for the interlayer insulating film is restricted in terms of hardness or elasticity, compared with prior art. As a result, there occurs a case where only an inferior material to conventional materials in terms of adherence and the like c an b e s elected to satisfy this point.
The following description discusses a preferred method of manufacturing a liquid crystal display device when, for example, the material for the interlayer insulating film does not have desirable adherence as mentioned above. A method of manufacturing a liquid crystal display device including: a pixel electrode and a switching element for determining a display state of a pixel; an interlayer insulating film disposed between the switching element and the pixel electrode; and a base substrate provided with the interlayer insulating film, the switching element and the pixel electrode preferably includes steps of: (1) providing the switching element on the base substrate; (2) providing the interlayer insulating film on the switching element; (3) improving film property by making an interlayer insulating film surface rough; and (4) providing the pixel electrode on the interlayer insulating film.
The third step includes, for example, a step of ashing treatment or light radiation in which the interlayer insulating film surface is made rough prior to the fourth step of providing the pixel electrode. As a result, even if a material of poor surface adherence is used as the resin forming the interlayer insulating film, the interlayer insulating film and the pixel electrode can adhere to each other with no trouble. Note that if the film property improving treatment is ashing treatment or light radiation treatment, the treatment can be carried out as follows. If the interlayer insulating film is photosensitive, the treatment can be carried out at the same time with a process of removing residue of the photosensitive resin, whereas if the interlayer insulating film is not photosensitive, the treatment can be carried out at the same time with a process of removing photoresist during patterning of the interlayer insulating film. This can avoid an increase of the number of manufacturing processes, compared with prior art.
Moreover, a method of manufacturing the above liquid crystal display device preferably includes steps of: providing the switching element on the base substrate; providing the interlayer insulating film on the switching element; drying the interlayer insulating film with prebaking treatment before patterning the interlayer insulating film; patterning the interlayer insulating film; and providing the pixel electrode on the interlayer insulating film.
With the configuration, the interlayer insulating film is dried with prebaking treatment before pattering the interlayer insulating film. Therefore, it is possible to prevent bad affection, such as dimensional distortion when the resin really cures, and to reduce viscosity of the resin forming the interlayer insulating film. Consequently, it is possible to improve productivity and dimensional accuracy in manufacture of the liquid crystal display device.
Moreover, the inventors of the present invention studied occurrence frequency of Newton rings with the above conventional liquid crystal display device, and confirmed among other things that Newton rings frequently occur in the sealing portion and that the rings were more clearly observed with a liquid crystal display device incorporating an interlayer insulating. film in an underlayer of the sealing portion.
On the other hand, in a preferred embodiment, the liquid crystal display device in accordance with the present invention includes: a liquid crystal layer; a first substrate provided on one side of the liquid crystal layer; and a second substrate having a base substrate facing the first substrate through the liquid crystal layer. On the base substrate of the second substrate, a sealing area is provided outside the display area, such as in the periphery of the base substrate. Moreover, an interlayer insulating film is provided between the base substrate and the liquid crystal layer, except in the sealing area. At least one switching element for controlling a display state of a pixel is provided on one side of the interlayer insulating film, whereas the pixel electrode is provided on the other side of the interlayer insulating film. In addition, the liquid crystal display device includes a sealing member, disposed to be in contact with the second substrate in the sealing area, for sealing the liquid crystal layer between the first and second substrates.
In the above configuration, the interlayer insulating film is not provided in the area where the sealing member and the second substrate are in contact with each other. Therefore, the space between the substrates can be prevented from varying when the sealing member cures. As a result, it is possible to prevent Newton rings to occur in a neighborhood of the sealing member. Besides, the defective ratio and reliability can be improved in manufacture of the liquid crystal display device.
As an addition to the above configuration, a second spacer is provided in the sealing member to maintain a thickness of the sealing member. Preferably, the second spacer satisfies at least one of the two conditions: the second spacer (1) is larger in size than the first spacer and (2) has a greater hardness than the first spacer. With this configuration, the space between the first and second substrates can be maintained with certainty. Therefore, the liquid crystal display device with high display quality can be realized.
In addition, in order to keep the space between the substrates constant, the liquid crystal display device preferably includes a middle film, provided on the sealing area of the base substrate, having better adherence than the base substrate. With this configuration, the adherence of the sealing member portion is improved, and the highly reliable liquid crystal display device can be obtained. Moreover, the middle film is preferably at least one film selected from the group consisting of a metallic film, a nitride film and an oxide film. In this case, the material for the middle film becomes the same with the material for the source wiring of the switching element and the like. Therefore, one process can be shared for forming the switching element and for forming the middle film, which is especially preferable.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.